1. Field of Invention
The present invention relates to a pointing device, and more particularly to a self-contained miniaturized smart self-calibration Electronic Pointing Device (EPD) for dismounted/mounted mortar weapons.
2. Description of Related Arts
The mortar's barrel pointing is a process of elevation and azimuth determination followed by pointing correction. The attitude determination and mortar pointing correction could be either separated in time sub-procedures or be a simultaneous process. The primary factors for the elevation and azimuth assignment are the coordinates of the mortar and the target on the map where the distance between the two points and the North direction is well defined. The mortar elevation is defined as an angle between its longitudinal axis and the earth's local horizontal plane. The mortar azimuth/heading is defined as an angle between the true North direction and the longitudinal axis counted from the North line in the horizontal plane (so that it complies with the map reading).
By definition, the azimuth is the bearing angle between the North direction (established by the line from the mortar base to the North) and the longitudinal axis of the mortar barrel (in a horizontal plane). The North direction is established on the map and/or physically by the two points S and N, as depicted in FIG. 3.
The mortar heading relates to its bearing angles with a simple formula:
      sin    ⁡          (              Azimuth        -                  B          N                    )        =                    L        SM                    L        SN              ⁢                  sin        ⁡                  (                                    B              N                        -                          B              S                                )                    .      With a certain correction on the elevation this relation allows considering the use of the mortar sight device for accurate pointing when the South-North baseline is physically established by GPS stands or natural markers designated on the local map.
The mortar's elevation is easy to determine using the local gravity vector's projection on the mortar barrel axes. Similarly the Earth rate projections give the heading of the mortar. Two physical vectors (gravity and Earth rate) are widely used in a heading elevation determination for stationary objects and this process is known as gyrocompassing.
Special attention is needed in considering the attitude determination and alignment with respect to the local vertical and north directions for moving objects. Changing a mortar base position with the carrier's movement (mounted mortar) alters the measurements. In general, we can not distinguish whether the attitude determination error/initial misalignment or the real movement causes the measurements changes. External information about the mortar's moves with respect to the earth (in addition to the inertial measurements) is needed. This information can be derived from the GPS aided INS and DGPS attitude determination system.
Significant improvements can be made by taking into account the different behaviours of the mortar movement with respect to the earth and unknown initial misalignment i.e. initial heading and elevation (these are unknown constants). In other words, the different spectra of the desired quantity (initial attitude) and of the mortar movements at low frequency allow estimating eventually the current attitude. In some cases when the motions are well modeled (with a restricted bandwidth in the frequency domain) we can perform the attitude determination in parallel with inertial sensor calibration owing to the mortar's movements.
Returning to the dismounted mortar we shall concentrate on the sensors calibration and error compensation. Since the mortar barrel movements are essential for the calibration process we introduce the indexing device (see chapter 2.6) for this purpose.